support django 2.1 test client json data automatically serialized

rest_framework/test.py

@@ -156,14 +156,19 @@ def _encode_data(self, data, format=None, content_type=None):
         Encode the data returning a two tuple of (bytes, content_type)
         """
 
-        if data is None:
-            return ('', content_type)
-
         assert format is None or content_type is None, (
             'You may not set both `format` and `content_type`.'
         )
 
         if content_type:
+            try:
+                data = self._encode_json(data, content_type)
+            except AttributeError:
+                pass
+
+            if data is None:
+                data = ''
+
             # Content type specified explicitly, treat data as a raw bytestring
             ret = force_bytes(data, settings.DEFAULT_CHARSET)
 
@@ -181,13 +186,17 @@ def _encode_data(self, data, format=None, content_type=None):
 
             # Use format and render the data into a bytestring
             renderer = self.renderer_classes[format]()
-            ret = renderer.render(data)
 
             # Determine the content-type header from the renderer
             content_type = "{0}; charset={1}".format(
                 renderer.media_type, renderer.charset
             )
 
+            if data is None:
+                ret = ''
+            else:
+                ret = renderer.render(data)
+
             # Coerce text to bytes if required.
             if isinstance(ret, six.text_type):
                 ret = bytes(ret.encode(renderer.charset))